We propose to study certain transport characteristics of the mammalian fetal and neonatal blood-gas barrier. The parameters we intend to measure for both the pulmonary capillary endothelium and the alveolar epithelium are the filtration coefficient and also reflection coefficients and permeability coefficients for low-molecular weight solutes. Measurements of these quantities will enable us to estimate equivalent pore radii for both membranes. We shall use an isolated, perfused lung preparation in these experiments, a preparation which has been used successfully by several workers, including us, to measure the indicated parameters in adult dog and rabbit lungs. By considering several different fetal and neonatal ages we hope to obtain a detailed picture of how the functional nature of the blood-gas barrier changes during development of the lung. In addition, we hope to use our data to help understand such phenomena as fluid secretion by the fetal lung and the rapid absorption of fluid from the lung in the early neonatal period.